Slip rake angles different than applied fault tractions in dynamic spontaneous rupture simulations

[tuxiang]

Hello, everyone

I am doing dynamic earthquake rupture simulation. I set the static coefficient, dynamic coefficient and effective normal stress as 0.6778, 0.525 and 120 MPa. I set the initial traction-shear-leftlateral as a Gaussian distribution, whose center is slightly larger than the static friction. The initial traction-shear-updip is set to 10 MPa, which is much smaller than the dynamic friction and static friction (Figure initial_stress.png). I think this initial parameters distribution will only cause strike slip. But the result shows that slip appears in both strike direction and dip direction. I do not know why.

Does anyone know the reason?
Best regards,

Tu

[baagaard]

Without a diagram of your problem setup, such as fault geometry in relation to the domain, etc, as well as a clear illustration and description of what you are seeing, it is impossible for us to tell whether this is a result of an error in setting up the problem or it is a consequence of the geometry of the problem.

[tuxiang]

Sorry, the detailed information about the dynamic rupture simulation test is below.
Firstly, I construct a simple geometry model. It is a planar embedded into the domain.

Secondly, I set the spatial database file. The material of the domain is set to be elastic. The surface of the domain is set to be free surface; other five boundaries are set to be absorbing boundaries.
I set the static coefficient, dynamic coefficient and effective normal stress as 0.6778, 0.525 and -120 MPa. I set the initial traction-shear-leftlateral as a Gaussian distribution, whose center is slightly larger than the static friction. The initial traction-shear-updip is set to 10 MPa, which is much smaller than the dynamic friction and static friction.

So the friction spatial database file is as following (here just list three columns):
#SPATIAL.ascii 1
SimpleDB {
num-values = 4
value-names = static-coefficient dynamic-coefficient slip-weakening-parameter cohesion
value-units = none none m Pa
num-locs = 3366
data-dim = 2
space-dim = 3
cs-data = cartesian {
to-meters = 1.0 // Specify coordinates in m for convenience.
space-dim = 3
} //cs-data
} // SimpleDB
// Columns are
// (1) x coordinate (m)
// (2) y coordinate (m)
// (3) z coordinate (m)
// (4) static-coefficient
// (5) dynamic-coefficient
// (6) slip-weakening-parameter (m)
// (7) cohesion (Pa)
-6000.000000 -11000.000000 0.000000 0.677800 0.525000 0.400000 0.000000
-5860.000000 -11000.000000 0.000000 0.677800 0.525000 0.400000 0.000000
-5720.000000 -11000.000000 0.000000 0.677800 0.525000 0.400000 0.000000

And the traction spatial database file is as following (here just list three columns):
#SPATIAL.ascii 1
SimpleDB {
num-values = 3
value-names = traction-shear-leftlateral traction-shear-updip traction-normal
value-units = MPa MPa MPa
num-locs = 3366
data-dim = 2
space-dim = 3
cs-data = cartesian {
to-meters = 1.0 // Specify coordinates in m for convenience.
space-dim = 3
} //cs-data
} // SimpleDB
// Columns are
// (1) x coordinate (m)
// (2) y coordinate (m)
// (3) z coordinate (m)
// (4) left-lateral shear traction (MPa)
// (5) reverse shear traction (MPa)
// (6) normal traction (MPa)
-3340.000000 -1523.076923 0.000000 71.327160 10.000000 -120.000000
-3200.000000 -1523.076923 0.000000 73.170816 10.000000 -120.000000
-3060.000000 -1523.076923 0.000000 74.935563 10.000000 -120.000000

Then I set the cfg files and run the simulation. Finally, I plot the slip.
The strike direction:

The dip direction:

Although the slip in dip direction is much smaller than that in strike direction, it slips in the dip direction.

[willic3]

Dear Tu,

After looking at your problem setup, one concern is that the outer boundaries of your domain may be too close to your fault. A larger domain may help. Also, what sort of boundary conditions are you applying to your outer boundaries?

Cheers,

Charles

[tuxiang]

Dear Charles,
The surface of the domain is set to be free surface; other five boundaries are set to be absorbing boundaries. Can I improve the results by adjusting the parameters of absorbing boundary? If so, how to adjust those parameters of absorbing boundary?
Best regards,
Tu

[baagaard]

For a dipping fault with low initial shear tractions, the dynamic (transient) shear traction changes on the fault can be large enough to cause rake angle rotations. This is well documented in the scientific literature (See Spudich, 1992, and studies citing that paper, such as Oglesby et al, 1999.

I strongly recommend becoming familiar with the breadth of scientific literature on spontaneous dynamic rupture simulations that has been published over the past 30+ years. A lot of these basic questions have been studied and documented and learning about them will save you a lot of headache in moving forward with your modeling. An easy way to find relevant studies is to look at the early papers cited in benchmarking studies and people who have been working in this area for a long time. See my earlier post for a list of people participating in the SCEC Dynamic Rupture Technical Activity Group.

[tuxiang]

Dear Baagaard,
Thank you very much.
Best regards,
Tu

[willic3]

Dear Tu,

I am not so familiar with dynamic problems, and it looks like Brad may have already provided a better answer. My comment about the proximity of the external boundaries to the fault is based on my experience with static problems. For dynamic
problems with absorbing boundaries, maybe this is not an issue.

Cheers,

Charles

[tuxiang]

Dear Charles,
Thank you all the same.
Best regards,
Tu